The invention relates to the casting of metal strip directly from a melt, and more particularly to the rapid solidification of an amorphous metal alloy directly from a melt to form substantially continuous metal strip.
The casting of very smooth strip has been difficult with conventional devices because gas entrapped as pockets between the quench surface and the molten metal during quenching form gas surface defects. These defects, along with other factors, cause considerable roughness on the quench surface side as well as on the opposite, free surface side of the cast strip. In some cases, the surface defects actually extend through the strip, forming perforations therein. Additionally, the uniformity of these surface defects across the width of a cast metal strip can vary.
U.S. Pat. No. 4,142,571 issued to M. Narasimhan discloses a conventional apparatus and method for rapidly quenching a stream of molten metal to form continuous metal strip. The metal can be cast in an inert atmosphere or a partial vacuum.
U.S. Pat. Nos. 3,862,658 issued to J. Bedell and 4,202,404 issued to C. Carlson disclose flexible belts employed to prolong contact of cast metal filament with a quench surface.
U.S. Pat. No. 4,154,283 to R. Ray et al. discloses that vacuum casting of metal strip reduces the formation of gas pocket defects. The vacuum casting system taught by Ray et al. requires specialized chambers and pumps to produce a low pressure casting atmosphere. In addition, auxiliary means are required to continuously transport the cast strip out of the vacuum chamber. Further, in such a vacuum casting system, the strip tends to weld excessively to the quench surface instead of breaking away as typically happens when casting in an ambient atmosphere.
U.S. Pat. No. 4,301,855 issued to H. Suzuki et al. discloses an apparatus for casting metal ribbon wherein the molten metal is poured from a heated nozzle onto the outer peripheral surface of a rotary roll. A cover encloses the roll surface upstream of the nozzle to provide a chamber, the atmosphere of which is evacuated by a vacuum pump. A heating element in the cover warms the roll surface upstream from the nozzle to remove dew droplets and gases from the roll surface. The vacuum chamber lowers the density of the moving gas layer next to the casting roll surface, thereby decreasing formation of air pocket depressions in the cast ribbon. The heating element helps drive off moisture and adhered gases from the roll surface to further decrease formation of air pocket depressions. The apparatus disclosed by Suzuki et al. does not pour metal onto the casting surface until that surface has exited the vacuum chamber. By this procedure, complications involved in removing a rapidly advancing ribbon from the vacuum chamber are avoided. The ribbon is actually cast in the open atmosphere, offsetting any potential improvement in ribbon quality.
U.S. Pat. No. 3,861,450 to Mobley, et al. discloses a method and apparatus for making metal filament. A disk-like, heat-extracting member rotates to dip an edge surface thereof into a molten pool, and a non-oxidizing gas is introduced at a critical process region where the moving surface enters the melt. This non-oxidizing gas can be a reducing gas, the combustion of which in the atmosphere yields reducing or nonoxidizing combustion products at the critical process region. In a particular embodiment, a cover composed of carbon or graphite encloses a portion of the disk and reacts with the oxygen adjacent to the cover to produce non-oxidizing carbon monoxide and carbon dioxide gases, which can then surround the disk portion and the entry region of the melt.
The introduction of non-oxidizing gas as taught by Mobley, et al., disrupts and replaces an adherent layer of oxidizing gas with the non-oxidizing gas. The controlled introduction of non-oxidizing gas also provides a barrier to prevent particulate solid materials on the melt surface from collecting at the critical process region where the rotating disk would drag the impurities into the melt to the point of initial filament solidification. Finally, the exclusion of oxidizing gas and floating contaminants from the critical region increases the stability of the filament release point from the rotating disk by decreasing the adhesion there between and promoting spontaneous release.
Mobley, et al., however, address only the problem of oxidation at the disk surface and in the melt. The flowing stream of non-oxidizing gas taught by Mobley, et al. is still drawn into the molten pool by the viscous drag of the rotating wheel and can separate the melt from the disk edge to momentarily disturb filament formation. The particular advantage provided by Mobley, et al., is that the non-oxidizing gas decreases the oxidation at the actual point of filament formation within the melt pool. Thus, Mobley, et al. fail to minimize the entrainment of gas that could separate and insulate the disk surface from the melt and thereby reduce localized quenching.
U.S. Pat. Nos. 4,282,921 and 4,262,734 issued to H. Liebermann disclose an apparatus and method in which coaxial gas jets are employed to reduce edge defects in rapidly quenched amorphous metal strips. U.S. Pat. Nos. 4,177,856 and 4,144,926 issued to H. Liebermann disclose a method and apparatus in which a Reynolds number parameter is controlled to reduce edge defects in rapidly quenched amorphous strip. Gas densities and thus Reynolds numbers, are regulated by the use of vacuum and by employing lower molecular weight gases.
U.S. Pat. No. 4,869,312 issued to H. Liebermann et al. discloses an apparatus and method for casting metal strip to reduce surface defects caused by the entrapment of gas pockets. A nozzle mechanism deposits a stream of molten metal within a quenching region of a quench surface to form a metal strip. A reducing gas is supplied to a depletion region located adjacent and upstream of the quenching region. The reducing gas reacts exothermically to provide a low density reducing atmosphere within the depletion region and to help prevent the formation of gas pockets in the strip.
Conventional methods, however, have been unable to adequately reduce the variation in surface defects across the width of a metal strip. Other shortcomings also exist in the prior art that are addressed and overcome by the present invention.
In one aspect, a method for casting continuous metal strip is disclosed. A chill body having a quench surface is moved at a selected speed, and a stream of molten metal is deposited on a quenching region of the quench surface to form the strip. Reducing gas is supplied to a depletion region located adjacent to and upstream from the quenching region. The reducing gas is provided by multiple gas nozzles, which may be separated from each other by baffles. A valve independently controls the flow of gas through each gas nozzle. The reducing gas is reacted exothermically to lower the density thereof and to provide a low density reducing atmosphere within the depletion region of each zone, independently. In a preferred embodiment, the metal strip is an amorphous metal alloy.
In a second aspect, a system is disclosed, which includes a casting surface such as a wheel, a molten metal supply, a reducing gas supply, a gas manifold including a plurality of independently controllable gas nozzles, and a plurality of gas flow control devices. The system provides for improved uniformity in the thickness profile of cast metal strip by allowing independent adjustment of gas flow to various regions in a depletion region. The system also provides for controlling both deleterious and advantageous ribbon surface features.
A third aspect includes an apparatus, which includes a casing with one open side, and several discrete compartments inside the casing separated by baffles. Each discrete compartment includes a gas nozzle. Gas nozzles are connected to a reducing gas supply via independently controllable valves. This arrangement allows the amount of gas flow to each discrete compartment to be controlled independently thereby providing a series of individual combustion chambers. This permits closer control of a strip""s thickness profile and surface features over specific areas of the metal strip.
Another aspect includes a method of controlling gas flow to various discrete sections of a quenching region in a metal strip casting system, which aspect includes using a sensor to evaluate the quality of a cast metal strip. This method of control permits automatic adjustment of the reducing flame atmosphere in various discrete sections of a quenching region independently.
The techniques disclosed advantageously minimize the formation and entrapment of gas pockets between the quenched surface and metal during the casting of metal strip and provide uniformity of strip thickness and uniformity of smoothness across the width of the strip.
There are other aspects of the invention that will be described herein.